Automatic gain control for frequency-hopped OFDM

ABSTRACT

An automatic gain control method and system for use in signal processing of OFDM symbols at a receiver. Two stages of coarse and fine automatic gain control are implemented that adjust different gains in an analog RF processing stage of the receiver. Gain of a low noise amplifier and a mixer are adjusted during a first and coarse automatic gain control stage based on feedback from a digital baseband stage. During a subsequent fine gain control period, the gain of a programmable gain amplifier is adjusted separately for each frequency band used by the OFDM symbols based on a histogram bin that counts the number of output samples of an analog to digital converter whose magnitude falls within certain ranges. Coarse and fine gains are updated after each OFDM symbol.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to the U.S. Provisional PatentApplication No. 60/709,542 filed on Aug. 18, 2005, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to ultrawideband (UWB)communications, and more particularly to automatic gain control (AGC) ina receiver of UWB communications.

UWB communication systems communicate information using what may beconsidered a large portion of frequency spectrum. For example, UWBsystems may use frequencies between 3.1-10.6 GHz. This portion of thefrequency spectrum may also be used by other communication systems.

Many proposed UWB systems are expected to use orthogonal frequencydivision multiplexing (OFDM). An OFDM carrier signal is the sum of anumber of orthogonal subcarriers. Baseband data on each subcarrier isindependently modulated. An example of OFDM symbol structure andfrequency hopping patterns are disclosed in Multiband OFDM PhysicalLayer Specification, Release 1.0, Jan. 14, 2005, which is incorporatedherein by reference.

A receiver of an OFDM UWB communication performs a variety of signalprocessing operations on the OFDM symbols received. The signal includingthe OFDM symbols may have range of levels or strengths over the specturmof the UWB communication. The signal processing steps or components mayeach impart a gain to the signal and the gain may vary depending on thefrequency of each portion of the signal. Changes in the input signallevel as a function of frequency may distort the signal and impairfurther processing steps. Appropriately setting the gain of a receivedsignal is therefore of some importance. Unfortunately, UWB transmissionsmay be bursty, or considered bursty in view of potential frequencyhopping, limiting time for performing automatic gain control (AGC)operations and also potentially increasing the difficulty of performingsuch operations.

SUMMARY OF THE INVENTION

The invention provides automatic gain control functions for a receiver,such as a wireless receiver. In one aspect the invention provides asystem for performing automatic gain control during signal processing ofOFDM symbols at a receiver, the system comprising a first stage adaptedto receive signals from a receiver antenna and configured to apply afirst gain to received signals; and a second stage including aprogrammable gain amplifier (PGA), the second stage coupled in serieswith the first stage, the second stage being adapted to provideamplified signals to an analog to digital converter (ADC), andconfigured to apply a second gain to signals generated by the firststage, the ADC being adapted to provide digitized output to a digitalbaseband processor, wherein the first stage is adapted to receive afirst gain setting signal, the first stage configured to adjust thefirst gain based on the first gain system signal; wherein the secondstage is adapted to receive a second gain setting signal, the secondstage configured to adjust the second gain based on the second gainsetting signal.

In another aspect the invention provides a method for adjusting a gainimparted to a signal received at a receiver, the method comprisinginitializing a first gain setting to a maximum and initializing a secondgain signal to a maximum; amplifying a received signal based on thefirst gain setting and the second gain setting; adjusting the first gainsetting based on an indication of received signal power; and adjustingthe second gain setting based on values provided by digitizing theamplified received signal.

In another aspect the invention provides A method for performingautomatic gain control during a signal processing of OFDM symbols in areceived signal at an RF receiver, the OFDM symbols arriving at the RFreceiver in different frequency subbands according to a patternindicated by a time-frequency code (TFC) for frequency subband hopping,the RF receiver having a first gain, a second gain, and a third gainoperating in series on the OFDM symbols, the third gain includingparallel gains, a number of the parallel gains being equal to a numberof the different frequency subbands, each parallel gain corresponding toone of the frequency subbands, each parallel gain having a gaincorresponding to a gain index, the method comprising initializing thefirst gain, the second gain, and the parallel gains of the third gain toa highest value for each; amplifying the OFDM symbols in all of thefrequency subbands by the first gain and the second gain, amplifying theOFDM symbols in each of the frequency subbands by the correspondingparallel gain; measuring an indication of a strength of the receivedsignal to obtain a received signal strength indication; obtaining afirst-second gain combination from a look-up table corresponding to thereceived signal strength indication; adjusting the first gain and thesecond gain by values corresponding to the first-second gaincombination; digitizing a result of the third gain to obtain digitizedoutputs; counting a first number of the digitized outputs havingabsolute values falling within a first range and a second number ofdigitized outputs having absolute values falling above the first range;changing the gain index if the first number and the second number exceedor fall below predetermined limits; and adjusting the third gain byadjusting each of the component gains to values corresponding to thechanged gain index.

These and other aspects of the invention are more fully comprehendedupon consideration of the following and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overview of a receiver in a communication systemaccording to the aspects of the present invention.

FIG. 2 shows an overview of a two-stage automatic gain control (AGC)system according to the aspects of the present invention.

FIG. 3 shows an exemplary architecture for an analog RF block accordingto the aspects of the present invention.

FIG. 4 shows a cascade of three component PGAs forming a PGA block thatis being used for received signal strength indicator (RSSI) measurementaccording to the aspects of the present invention.

FIG. 5 is a flow diagram of a process for performing AGC in accordancewith aspects of the invention.

FIG. 6 is a flow diagram of a further process for performing AGCaccording to the aspects of the present invention.

FIG. 7 is a diagram showing an exemplary OFDM symbol structure and anexemplary frequency hopping pattern for the OFDM symbols.

FIG. 8 shows an exemplary packet format for a signal including OFDMsymbols.

FIG. 9A shows a sequence of events for a packet format having a longpreamble for various time-frequency hopping patterns used during signalprocessing according to the aspects of the present invention.

FIG. 9B shows a sequence of events for a packet format having a shortpreamble for various time-frequency hopping patterns used during signalprocessing according to the aspects of the present invention.

FIG. 10A shows a two-bin histogram for fine AGC according to the aspectsof the present invention.

FIG. 10B shows the number of output samples of an analog to digitalconverter (ADC) exceeding certain threshold values versus time accordingto the aspects of the present invention.

FIG. 11 shows an exemplary state machine representation for the signalprocessing part of a receiver according to the aspects of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows an overview of a physical layer for a receiver in acommunication system according to the aspects of the invention. Thereceiver of FIG. 1 includes an analog RF block 110 and a digitalbaseband block 111 that are coupled through an analog to digitalconversion (ADC) block 112. A signal transmitted by a transmitter andreceived by the receiver at an antenna 101 is in analog form. The analogRF block 110 amplifies the signal and downconverts it from radiofrequency to baseband. The ADC block converts the baseband analog signalto a digital signal. The digital baseband block 111 further processesthe signal, and provides the processed signal to a MAC 141.

In one embodiment, the receiver shown in FIG. 1 may receive at thereceiver antenna OFDM symbols. The OFDM symbols are processed throughthe analog RF processing block 110. The analog RF processing block 110may include an amplification block and a downconversion block, foramplification and downconversion from RF to baseband. Duringdownconversion by the analog RF block 110, a frequency hopping patternof the OFDM symbols may be taken into account. The frequency hoppingpattern may be determined according to a time-frequency code (TFC)number. The hopping pattern is provided to the downconversion process bya media access controller (MAC).

After analog RF processing the signals are digitized by the ADC block112. The ADC block 112 is coupled to the digital baseband processingblock 111, which includes a signal processing block 120 followed by anumber of data processing blocks that further process the data resultingfrom the signal processing part before the data is provided to the MAC141. As illustrated in FIG. 1, the digital baseband block includes asignal processing packet detection, frame synchronization and automaticgain control (AGC) determination block 120. A null prefix removal,overlap and add, and DC offset compensation block 124, and an FFT,demapping, and decoding block 132. In many embodiments each of theseblocks may themselves include blocks, for example the FFT, demapping,and decoding blocks may each be implemented as their own blocks,generally in circuitry, but as programming in a processor in someembodiments.

In many embodiments the receiver is a receiver for a packet-basedcommunication system, preferably for OFDM time frequency hoppingcommunications. In such receivers, and receivers of other embodiments,AGC may be performed to adjust the amplification of the received signal.For example a received signal strength indication (RSSI) signal may begenerated by the analog RF block 110. AGC determination circuitry mayuse an analog-to-digital converted version of the RSSI as a measure ofthe RF power of the received signal. An appropriate control signal maybe generated during the processing by the signal processing block 120and fed back for use by the analog RF processing block 110 to controlgains imparted to the received signal. AGC may include controlling thegains of several components. In some embodiments, AGC may include thecomputation of gain settings for a low-noise amplifier (LNA), a mixer,and a programmable gain amplifier (PGA).

FIG. 2 is a block diagram of portions of a receiver including two-stageAGC operation in accordance with aspects of the invention. An analogprocessing block 200, generally including RF analog circuitry, may beconsidered as including a coarse AGC portion 210 and a fine AGC portion220. In many embodiments the course AGC portion includes a low noiseamplifier (LNA) and a mixer, and the fine AGC portion includes anamplifier chain including at least one programmable amplifier. Thecoarse AGC portion 210 receives an input signal from an antenna 201 andprovides a gain to the signal based on a course AGC gain signal 232,provided by a digital baseband block as illustrated in FIG. 2. The fineAGC portion 220 receives the signal from the coarse AGC portion. Thefine AGC portion also provides a gain to the signal, with the gaindependent on a fine AGC signal provided by the digital baseband. Outputof the fine AGC portion is converted from analog format to digitalformat by an ADC block 270.

In some embodiments, the two gain stages, course AGC and fine AGC, areused to control the input to the ADC block such that clipping of thesignal amplitude by the ADC is kept at a predetermined level. Thus, thefeedback control signals 232, 233 may be determined by the digitalbaseband based on signal levels provided by the ADC. In manyembodiments, however, and as illustrated in FIG. 2, an RSSI signal 221is provided by the analog processing block. The digital baseband may usethe RSSI to determine AGC gain settings, either alone or in conjunctionwith other signals, such as outputs of the ADC.

A UWB signal received by the antenna 201 may include several frequencysubbands. When the received signal includes several frequency subbands,a band-select signal 232 from the digital baseband stage may be used bycircuitry of the coarse AGC portion, as described including for examplea mixer or multiple mixers, to select the subbands that are to bedownconverted from RF to baseband frequency. The band-select signal maybe based on a frequency hopping pattern indicated by a TFC number. Insome embodiments gain settings for the course AGC portion or the fineAGC portion, or both, may use different gain settings based on a subbandindicated by the band-select signal. For example, the fine AGC stage mayinclude separate components corresponding to each frequency subband suchthat a different gain may be implemented for each subband. Also forexample, the fine AGC portion may include parallel PGAs, each parallelPGA amplifying or attenuating a signal transmitted on a particularfrequency subband.

FIG. 3 shows an exemplary architecture for an analog RF block inaccordance with the aspects of the invention. In some embodiments, theanalog RF block is used in a receiver such as the receiver of FIG. 1.The analog RF block of FIG. 3 includes a first stage AGC portion 310 anda second stage AGC portion 320 coupled together in series. The firststage AGC portion 310 provides a coarse AGC gain and the second stageAGC portion 320 provides a fine AGC gain to a received signal. The firststage AGC portion receives a signal 301 from a receiver antenna. Asillustrated in FIG. 3, the first stage AGC portion includes an LNA 340coupled to a mixer 350.

The signal 301 from the receiver antenna is amplified by the LNA 340,with the gain dependent on a course AGC signal 332. In oneimplementation, the amplification gain is controllable to levels of 10dB, 21 dB and 27 dB. The LNA is coupled to the mixer 450. The mixerdown-converts the received signal baseband. In most embodiments themixer inherently includes a gain, although the gain is shown as aseparate amplifier 355 in FIG. 3 for convenience. In one implementation,the mixer gain of the mixer 350 can be controlled to levels of −13 dB,−7 dB, −1 dB, and +5 dB. The −13 dB, −7 dB, and −1 dB levels areattenuations, but for convenience both attenuation and amplification,such as +5 dB amplification, are both generally simply referred to as“amplification.”

As illustrated, the mixer 350 performs down-conversion of varioussubbands from RF to baseband. For example, if the received signal isexpected to be provided at three frequency subbands, with a particularsub-band used at any instance, the mixer 350 may include three mixer351, 352, 353 each mixing the received and amplified RF signal with amixing signal of a different frequency, with an output of the variousmixers selected using the band-select signal 331.

For example, communications received by a receiver may include OFDMsymbols that are transmitted over multiple frequency subbands. Eachsymbol may occupy a different subband and the consecutive OFDM symbolsmay hop from subband to subband according to a time-frequency code (TFC)that is assigned a TFC number. Table 1 shows exemplary frequency hoppingpatterns with time or TFCs. Each pattern is assigned a TFC number andshows the frequency subbands occupied by consecutive OFDM symbols. TABLE1 Examples of time-frequency codes (TFCs) for band group 1 of a UWBcommunication system that ranges from 3.168 GHz to 4.752 GHz. TFC k = 0k = 1 k = 2 k = 3 k = 4 k = 5 1 1 2 3 1 2 3 2 1 3 2 1 3 2 3 1 1 2 2 3 34 1 1 3 3 2 2 5 1 1 1 1 1 1 6 2 2 2 2 2 2 7 3 3 3 3 3 3

In Table 1, the leftmost column shows the TFC numbers or TFC logicalchannels ranging from 1 to 7. Subsequent columns show the frequencysubbands used for transmission during each normalized time k for k=0through k=5. Each normalized time corresponds to one OFDM symbol that istransmitted during one subband. The hopping period for each of the sevenTFC schemes shown in Table 1 is therefore six OFDM symbols. However,within the period of six symbols, some patterns begin repeating withinfewer symbols. For example, with TFC numbers of 1 and 2, the hoppingpatterns repeat after 3 symbols.

The second stage AGC portion includes an amplifier such as aprogrammable gain amplifier (PGA) 360. In most embodiments, however, theamplifier is an amplifier chain including a plurality of programmablegain amplifiers and a plurality of amplifiers with a preset gain. ThePGA receives a baseband signal from the mixer, amplifies the signal, andprovides the amplified signal to an ADC 370. The ADC output 371 isintended for use by a digital baseband processor.

The PGA gain is based on a fine AGC signal 333. PGA 360 can implementamplification from 6 dB to 30 dB in steps of 2 dB, as later discussed.In some embodiments, when the received signal may be at differentfrequency subbands, the PGA 360 has different gain settings for eachsubband.

In one embodiment all initial gains are set to their respective maximalvalue, for example to increase detection of very weak signals. Forexample, the LNA gain is set at 27 dB, the mixer gain is set at 5 dB,and the PGA gain is set at 30 dB, in order to set the receiver tomaximal sensitivity. These values thereafter may be refined for exampleto regulate the clipping of the ADC output to a desirable value.

In one embodiment, the two stages of AGC control the amplitude of theinput to ADC such that only about 5% of the ADC output samples areclipped to maximum absolute value.

In some embodiments, and as illustrated in FIG. 3 an RSSI signal 361 isproduced by the PGA. The RSSI signal, usually after conversion todigital form, may be used to estimate the RF power (P_(RF)) of thereceived signal 301. For example, the received RF power P_(RF) imay becomputed by the digital baseband processor based on LNA gain, mixer gainand PGA gain, may be averaged over one OFDM symbol in digital baseband,using a sliding window. Further, an estimate of the received signalpower P_(RSSI) may be obtained using the current PGA gain settings asthreshold values and compared to the averaged versions of the RSSImeasurement result to obtain an estimate of the received RF power P_(RF)from the antenna may be computed as:P _(RF)[dB]=P _(RSSI)[dB]−LNA _(gain)[dB]−mixer_(gain)[dB]−P _(RSSI)_(—) _(BIAS)[dB]Where P_(RSSI) _(—) _(BIAS)[dB] is a correction term, if needed oruseful, and often implementation dependent.

The received RF power P_(RF) may be used, for example, as an index to alook-up table which stores LNA and mixer gain combinations. Depending onthe current estimate of P_(RF), new LNA and mixer gains may bedetermined, and fed back to the analog RF part, as the coarse AGCgain-setting signal 432, and may also be used for implementations whereP_(RF) is calculated at the PGA instead of P_(RSSI).

FIG. 4 shows a cascade of three component PGAs forming a PGA block 420.In addition, an RSSI measurement block 460 is also shown. The PGA block420 shown in FIG. 4 may correspond to the PGA 360 of FIG. 3.

Each PGA block 420 of a fine AGC stage may be composed of a cascade ofthree separate component PGAs 510, 415, 530 that can be separatelycontrolled by the fine AGC gain-setting digital baseband signal orsignals. Each of the separate component PGAs in the cascade has aprogrammable gain, for example of 2, 4, 6, 8, or 10 dB. A control signal433 from the digital baseband is used to set the gain of each componentPGA, with each of the gains separately settable. Outputs from each ofthe separate component PGAs is provided to the RSSI measurement unit460, using sampling circuitry in some embodiments. An RSSI signal 461 isobtained from the RSSI measurement unit 460, for provision to thedigital baseband stage.

The cascade of PGAs may be used for two purposes: to amplify a receivedsignal and to derive the RSSI 461. The RSSI 461 may be an estimate ofthe received signal power P_(RF) in dBm. Power in units of dBm isrelated to power in units of Watts according to the relationshipP[dBm]=10.log₁₀ (P[Watts]/1 mW)). The RSSI 461 is obtained by usingsignals x₁, x₂, x₃, and x₄ that are sampled at the output of eachcomponent PGA 410, 420, 430. The RSSI 461 may be an analog signal whichis analog-to-digital converted for further processed in digitalbaseband.

Various combinations of gains for each of the three component PGAs maybe used to obtain overall gain for the PGA block. A gain index may beused to determine which combination of component gains is to be used.The gains of component PGAs are added together to set the overall gain.Table 2 shows the PGA gain settings for each of the three component PGAs410, 415, 430 required to achieve an overall PGA gain from 6 dB to 30dB. Table 2 may be used to the fine AGC signal. TABLE 2 Gain settingsfor component PGAs to achieve various overall PGA gains. PGA gain gain[dB] gain [dB] gain [dB] Overall gain [dB] index PGA 1 PGA 2 PGA 3PGA1-3 0 2 2 2 6 1 4 2 2 8 2 6 2 2 10 3 8 2 2 12 4 10 2 2 14 5 10 4 2 166 10 6 2 18 7 10 8 2 20 8 10 10 2 22 9 10 10 4 24 10 10 10 6 26 11 10 108 28 12 10 10 10 30

As shown in Table 2, A PGA gain index varies between 1 and 12. The gainindex may be used to set the gain for the three component PGAs. The gainin dB of each of the component PGAs 410, 420, 430 are shown insubsequent columns, with the overall gain of the three cascadedcomponent PGAs is shown in the rightmost column. The gain of each of thecomponent PGAs can take values 2, 4, 6, 8, or 10. The overall gain isobtained by adding the gains of the three components and varies from 6to 30 dB in increments of 2. For example, to obtain an overall gain of 6dB, each component PGA may have a gain of 2 dB. To obtain an overallgain of 30 dB, the maximum overall gain in one embodiment, and used asan initial value of gain for the PGA, each component PGA is set to havea gain of 10 dB.

FIG. 5 is a flow diagram of a process for performing AGC. Preferably AGCis performed during reception of a preamble portion of a packet. Inblock 502 the process of a coarse AGC gain and a fine AGC gain areinitialized. In one embodiment the gains are set to maximum values,which may be accomplished by setting both a course AGC gain signal and afine AGC gain signal. This may be done, for example to increase thesensitivity of detection of the signals arriving at the receiver to amaximum.

In block 504 the process determines an indication of received signalstrength. In some embodiments this is accomplished by determining anRSSI signal. In some embodiments this is accomplished by extracting anintermediate signal within an RF processing block, and subtracting outgains expected from components in the signal processing chain prior toextraction. For example, the signal may be extracted subsequent to theLNA and mixers, and gains associated with these elements are subtractedfrom a strength of the extracted signal.

In block 506 the process performs a coarse AGC. In many embodiments thiscomprises selecting a gain level based on the indication of receivedsignal strength and providing the selected gain level to at least somegain elements in the RF processing block. In many embodiments the gainelements receiving the selected gain level, which may be considered acoarse AGC gain level, are the LNA and the mixer or mixers.

In block 508 the process performs a fine AGC. In many embodiments adigital baseband receives outputs of an ADC, evaluates the level oraverage level of the outputs of the ADC, and selects a fine AGC gainlevel based on the evaluation. Preferably the gain level is selectedsuch that no more than 5% of the ADC outputs are at a maximum or minimumfor the ADC outputs. The gain level is provided to gain elements of theRF processing block, preferably programmable gain elements of anamplifier chain. The process performed in block 508 is in manyembodiments an iterative process repeated over several consecutivereceived symbol periods.

The process thereafter returns.

FIG. 6 is an embodiment of a process for performing fine AGC. In block611 the process compares ADC outputs to various thresholds, which may bepreset or programmable by way, for example, of external registers. Inblock 613 the process updates counters based on whether the ADC outputsexceed, or in some embodiments do not exceed, the thresholds. In block615 the process compares the counters to further thresholds. In block617 the process updates the gain settings, preferably just the AGC finegain settings, based on the result of the comparison of the counters tothe further thresholds. The process thereafter returns.

The process of FIG. 6 is more fully comprehended considering the exampleof FIG. 10 a and FIG. 10 b. FIG. 10A shows a two-bin histogram for usingwith counters and FIG. 10B shows the number of ADC output samplesexceeding certain threshold values versus time.

For fine tuning the fine AGC stage, output samples of the ADC block maybe examined to determine if the magnitudes of the output samples satisfya desirable criteria. If too many of the output samples are consideredto have an undesirably large magnitude, then the gain of the AGC may bereduced. A reduced AGC gain results in a smaller analog input to the ADCand smaller ADC output samples. If the output samples happen to haveundesirably small magnitudes, then a feedback to the fine AGC stageincreases the gain and adjust the output of the AGC stage upward.

The desirable criteria may be a range, a maximum value not to beexceeded, or a minimum value not to fall below. One method of evaluatingthe output samples of the ADC block is to form a histogram of themagnitudes of the output samples that counts the number of outputsamples falling within various ranges. Then, different criteria may beapplied to different ranges of the histogram. One embodiment of theinvention, uses a two-bin histogram to classify the output samples ofthe ADC block according to their magnitude.

A two-bin histogram 1010 is used to count how many of the ADC outputsamples have absolute values between two specified threshold values a1,a2 and how many have absolute values exceeding both limits, preferablyduring a sliding window of one OFDM symbol. The sliding window of oneOFDM symbol may, for example, correspond to the time it takes for one ofthe OFDM symbols 701-704 of FIG. 7 to be received at the receiver. Theabsolute values of the output signals from the ADC correspond to thereal component of the complex output signals. In FIG. 10, a1<a2. Thecorresponding counted values are c1, c2 such that the ADC output samplesexceeding the threshold a2 are counted in bin c2 and the samplesexceeding the threshold a1, but smaller than a2, are counted in bin c1.The thresholds a1, a2 can be provided by external register settings.

In plot 1020, the values y of the ADC output signals are plotted versustime. A max and a min value are marked on the plot 1020. The ADC outputsignals having a value y above the max or below the min are clipped bythe ADC. Therefore, it is desirable to lower the PGA gain such that thesymbol entering the ADC is not clipped while keeping the gain highenough to maintain sensitivity. Hence, the histogram 1010 counts the ADCoutputs exceeding the threshold values a1 and a2 to keep the totalnumber of those that are too high or too low few by fine-tuning the PGAgain.

The fine AGC stage may performed for each subband separately. Therefore,for each frequency subband, such as subband 1, subband 2 and subband 3,a separate two-bin histogram is maintained.

Depending on the two-bin histogram 1010 measurements, the gain isincreased or decreased in subsequent fine AGC steps after the initialstep.

One exemplary way of changing the PGA gain index 350 of Table 2depending on the histogram measurements 1010 is as follows: IF(COUNT1<c2≦2×COUNT1) THEN gain_index=gain_index−1; (slow decrease) ELSEIF (c2>2×COUNT1) THEN gain_index=gain_index−2; (fast decrease) IF(0.5×COUNT2<cl≦COUNT2) THEN gain_index=gain_index+1; (slow increase)ELSE IF (c1<0.5×COUNT2) THEN gain_index=gain_index+2; (fast increase)

Where COUNT1 and COUNT2 can be provided by external registers. COUNT1 isa limit selected for the bin collecting the ADC outputs between a1 anda2 and COUNT2 is a limit selected for the bin collecting the outputsexceeding a2.

FIG. 7 is a diagram showing an exemplary OFDM symbol structure and anexemplary frequency hopping pattern for the OFDM symbols. As shown inFIG. 7, the OFDM symbol includes a predetermined number of samples thatare divided into several categories or groups. Some groups of thesamples include data being transmitted. Other groups of samples do notinclude any data or include a repetition of earlier data and are used toincrease process ability to the OFDM symbol. Other groups of samples areused to guard against the effects of interference between symbols thatarrive successively through different channels and are subject to pathdispersion. Also, shown in FIG. 7 is the frequency hopping pattern ofthe OFDM symbols. The symbols may stay in the same frequency band or mayhop from band to band according to a pattern.

Thus, the exemplary plot of FIG. 7 shows the frequency subbands used fortransmission of OFDM symbols 701, 702, 703, 704 versus normalized time.An exemplary OFDM symbol such as a third OFDM symbol 703 of FIG. 7includes a fast Fourier transform (FFT) window, a null prefix or postfix(NL), and a guard interval (GI). In digital baseband, each OFDM symbolincludes 165 complex samples. With an exemplary sampling frequency of528 MHz=1/Ts, the number of samples usable for a FFT operation, or theFFT window size, is N_(FFT)=128 samples. The null prefix, which isactually a postfix in the OFDM symbol shown, has a length of N_(NL)=32samples that are set to zero at the transmitter. The guard intervalsamples have a length of N_(GI)=5 samples.

All OFDM symbols may be transmitted on a same frequency or, as shown inFIG. 7, each OFDM symbol may be transmitted on a different frequency. Afrequency-hopped OFDM may be implemented where the center frequency ofthe transmitted OFDM symbol is changed for every OFDM symbol accordingto a frequency hopping pattern determined by a TFC number. This isreferred to as time-frequency interleaving. The frequency hopping occurstoward the end of the OFDM symbol during the guard interval N_(GI). Forexample in FIG. 7 frequency hopping occurs at or around discretenormalized time k=160 and during the 5 last samples within the guardinterval. The transient effects, resulting from the switching of RFfrequency, are supposed to happen during the guard interval. Thus, theN_(GI) samples of the guard interval are invalid and are skipped not tobe further used in the digital baseband processing.

In FIG. 7, three frequency subbands, subband 1, subband 2, and subband 3are shown on the vertical axis. The normalized time shown on thehorizontal axis is obtained as k=t/T_(S) where t is time in seconds,T_(S) is the sampling period in seconds, and the normalized time k isdimensionless. A first OFDM symbol 701 is transmitted on the subband 1,a second OFDM symbol 702 is transmitted on subband 2, a third OFDMsymbol 703 is transmitted on subband 3, and a fourth OFDM symbol 704 istransmitted on subband 1 again with the same frequency hopping patternrepeating thereafter. The frequency hopping scheme of FIG. 7 is subband1, subband 2, subband 3, subband 1 again, and repeating thereafter. Thisscheme corresponds to the TFC number 1 in Table 1.

FIG. 8 shows an exemplary packet format 800 for a signal including OFDMsymbols. Transmission of OFDM symbols may be organized in packets andeach packet may contain many OFDM symbols. A packet may include apreamble, a header and a payload part. In the preamble, a predeterminednumber of OFDM symbols, that are transmitted first, form a packetsynchronization sequence that may be used for packet detection, framesynchronization, and AGC. The remaining symbols of the preamble form achannel estimation sequence that may be used for demapping at areceiver. The payload part includes the data.

In a packet-based communication system AGC may be typically performedduring the receiving of the preamble, with dedicated time-windows forperforming the gain adjustments of the different stages of the RF part.

An exemplary packet 800 comprises a preamble 810, a header 820 and apayload part 830. The preamble 810 may be long including 30 OFDM symbolsor short including 18 OFDM symbols. In the preamble 810, the first 24OFDM symbols, for a long preamble, or the first 12 OFDM symbols, for ashort preamble, form a packet synchronization sequence 811. The packetsynchronization sequence 811 is used for packet detection, framesynchronization, and AGC. The remaining 6 symbols of the preamble 810form a channel estimation sequence 812 whose symbols are used to performchannel estimation. The channel estimates are used for coherentdemapping in the data processing block 120 of the receiver of FIG. 1.The header 820 may contain 6 OFDM symbols. The payload part 830 includesthe data and contains as many OFDM symbols as required to convey thenumber of bytes in the payload.

FIG. 9A shows a sequence of events for a packet format having a longpreamble 900 for various time-frequency hopping patterns used duringsignal processing. FIG. 9B shows a sequence of events for a packetformat having a short preamble 950 for various time-frequency hoppingpatterns used during signal processing. The time-frequency hoppingpatterns are identified by the corresponding TFC number shown in Table1.

A data packet may have a long or a short preamble. In some embodimentpackets having a long preamble are treated differently from the packetshaving a short preamble for AGC purposes. For example, short preamblesgenerally are used as a part of data bursts where the first packetincludes a long preamble that may be used to perform a coarse AGC. Theresults of the coarse AGC of the first packet in the burst is laterapplied to the packets following the first packet if they happen toinclude a short preamble. Thus, the symbols of a long preamble may beused to first perform a coarse AGC and then to perform a fine AGC. Whena packet arrives having a short preamble, only a fine AGC is performed,with the results of the coarse AGC performed on a previous packetapplied to the packet having the short preamble.

The fine AGC may also operate differently on packets that are receivedon different time-frequency hopping patterns corresponding to differentTFC numbers. When successive OFDM symbols hop from one frequency band toanother, corresponding to TFC1-TFC4, then a larger number of symbols maybe used for performing the fine AGC operation. When all of the symbolsare arriving on the same frequency subband, corresponding to TFC5-TFC6,then a fewer number of symbols may be used for performing the fine AGCoperation. The coarse AGC is capable of using the same number of symbolswhether or not different frequency bands are used by the symbols. Thegains are updated at the end of each OFDM symbol during the coarse AGC.After the coarse AGC stage has determined the gains of the componentsused in the coarse AGC portion, then fine AGC is performed. The fine AGCstage fine-tunes the overall gain per subband.

FIGS. 9A and 9B each include the 7 frequency hopping schemescorresponding to TFC numbers of 1 through 7 shown in Table 1. For TFCnumber equal to 1, the frequency bands used by successive OFDM symbolshop from 1 to 2 to 3 and then back to 1. For TFC number equal to 2, thefrequency bands used by successive OFDM symbols hop from 1 to 3 to 2 andthen back to 1. For TFC number equal to 3, the frequency bands used bysuccessive OFDM symbols hop from 1 to 1 to 2 to 2 to 3 to 3 and thenback to 1 and 1 repeating the same double hop pattern. For TFC numberequal to 4, the frequency bands used by successive OFDM symbols hop from1 to 1 to 3 to 3 to 2 to 2 and then back to 1, repeating the same doublehop pattern. TFC numbers 5, 6, and 7 correspond to no frequency bandhopping. At TFC number equal to 5, successive OFDM symbols stay infrequency subband 1. At TFC number equal to 6, successive OFDM symbolsstay in frequency subband 2. At TFC number equal to 7, successive OFDMsymbols stay in frequency subband 3.

FIG. 9A shows the schedule of a coarse AGC 910 for a packet having thelong preamble 900. For all TFC values 1-7 that are listed in Table 1,the OFDM symbols with index 2 and index 3, i.e. those that use thefrequency subbands 2 and 3 as shown in FIG. 7, are used to perform thecoarse AGC. The gains are updated at the end of each OFDM symbol duringthe coarse AGC 910.

FIG. 9B shows the schedule for a packet having the short preamble 950.No coarse AGC is performed for this type of packet. Instead, the coarseAGC settings are kept from a previous packet. A short preamble is onlyused for packets that are part of a burst of packets, where the firstpacket always has a long preamble that is used to determine coarse AGCgain settings. Subsequent packets of the same burst, optionally mightuse a short preamble for reduced overhead and, in turn, higherthroughput. A receiver physical layer, such as the physical layer of thereceiver of FIG. 1, is told by a MAC whether to expect a long or a shortpreamble for the next incoming packet.

In both FIGS. 9A and 9B, only portions that correspond to the packetsynchronization sequence 811 part of the preamble 810 in FIG. 8 areshown. The first sample in each packet is used for packet detection 901,951. In FIG. 9A for the packet with the long preamble, a timingacquisition window 902 follows the first sample and a packetconfirmation window 903 follows the second sample. However, for a packetwith the short preamble of FIG. 9B, only a packet confirmation window953 follows the first symbol.

After the coarse AGC 910, FIG. 9A includes a delay 911 of one sample.FIG. 9B does not include a coarse AGC or a subsequent delay. Both FIGS.9A and 9B include fine AGC periods 920, 940, 970, 990 followed by framesynchronization 921, 971 and finally channel estimation 922, 972 afterthe long or short preamble 900, 950 samples have been exhausted.

The TFC5-TFC7 hopping patterns that correspond to no frequency bandhopping, have a shorter fine AGC 940, 990 period than the TFC1-TFC4which involve actual frequency band hopping. As a result, the framesynchronization periods 921, 971 of these TFC patterns are longer.

During the fine AGC stage, the PGA gains of each of the subbands areupdated at the end of each OFDM symbol. The schedule for the fine AGCstage 920, 940, 970, 990 depicted in FIG. 9A for the packet 900 having along preamble and in FIG. 9B for the packet 950 having a short preamble,takes into account the frequency hopping pattern for the particular TFCused.

With respect to FIG. 9A, after the coarse AGC 910, a delay of one OFDMsymbol 911 allows the analog circuitry in the RF part to settle on thenew LNA and mixer gain settings obtained by the coarse AGC stage.Because the fine AGC 920 is performed per subband, the particularfrequency hopping pattern depending on the TFC number needs to be takeninto account. The period of the hopping patterns is 6 OFDM symbols asindicated in Table 1. For TFC1-TFC4, the fine AGC 920 is performed over6 OFDM symbols, with 2 OFDM symbols having the same subband. Then eachsubband occurs twice during the fine AGC stage 920 and the PGA gain foreach subband is updated two times during the fine AGC stage 920. Theupdate of the PGA gain occurs as explained in the description of FIG. 4above and FIG. 10A and FIG. 10B below. For TFC5-TFC7, the fine AGC 940is performed over 3 OFDM symbols with all 3 OFDM symbols having the samesubband. Then, the subband corresponding to the TFC group occurs threetimes during the fine AGC stage 940 allowing to update the PGA gainsthree times for the single subband used.

For example, for TFC6 all of the OFDM symbols use the same frequencysubband 2. Further, during the fine AGC 940 the PGA gains of thesubbands being used are updated at the end of each OFDM symbol. Thesecond frequency subband, subband 2, is therefore used three timesduring the fine AGC stage 940 to transmit all three symbols. As aresult, the gain of the PGA, such as PGA 520, is updated three timesduring the fine AGC stage 940.

With respect to FIG. 9B, no coarse AGC is performed for the shortpreamble packet. Instead, the coarse AGC settings of the previous packetare carried over. The schedule for the fine AGC 970, 990 in this case isidentical to the long preamble processing. Therefore, 6 OFDM symbols areused for TFC1-TFC4, and 3 OFDM symbols are used for TFC5-TFC7. The PGAgain is, therefore, updated 3 times for each frequency subband 1 through3, during the fine AGC stage 970, 990 for a packet having a shortpreamble as well.

FIG. 11 shows an exemplary finite state machine (FSM) representation fora signal processing part 1120 and a data processing part 1130 that mayrespectively correspond to the signal processing and data processingparts of the receiver of FIG. 1. During the signal processing part 1120,the FSM controls the orchestration of the different elements. The FSMincludes dedicated states for packet detection 1101 and confirmation1102, coarse and fine automatic gain control 1103, 1105 and framesynchronization 1106, that correspond to the signal processing part1120. A coarse AGC delay state 1104 is also located between the coarseAGC state 1103 and the fine AGC state 1105. After the signal processingpart 1120 is complete, control is handed over to the data processingpart 1130 which takes care of FFT, demapping, deinterleaving and Viterbidecoding. Any of the states may revert to an idle state 1110 after dataprocessing or signal processing operations are complete.

Although the invention has been described with respect to certainspecific embodiments, it should be recognized that the inventioncomprises the claims and their equivalents supported by this disclosureand insubstantial variations thereof.

1. A system for performing automatic gain control during signalprocessing of OFDM symbols at a receiver, the system comprising: a firststage adapted to receive signals from a receiver antenna and configuredto apply a first gain to received signals; and a second stage includinga programmable gain amplifier (PGA), the second stage coupled in serieswith the first stage, the second stage being adapted to provideamplified signals to an analog to digital converter (ADC), andconfigured to apply a second gain to signals generated by the firststage, the ADC being adapted to provide digitized output to a digitalbaseband processor, wherein the first stage is adapted to receive afirst gain setting signal, the first stage configured to adjust thefirst gain based on the first gain system signal; wherein the secondstage is adapted to receive a second gain setting signal, the secondstage configured to adjust the second gain based on the second gainsetting signal.
 2. The system of claim 1 wherein the first stagecomprises a first amplifier coupled in series to at least one mixer, andwherein the second stage comprises at least one programmable gainamplifier (PGA).
 3. The system of claim 2, wherein the PGA includes: aplurality of component amplifiers coupled together in series, eachamplifier being adapted to receive at least a component of the secondgain setting signal and to provide a gain based on the component secondgain setting signal.
 4. The system of claim 3, further comprising anevaluation unit coupled to inputs and outputs of at least some of thecomponent amplifiers and the evaluation unit being configured formeasuring an indication of received signal strength received signalsduring a sliding time window.
 5. The system of claim 4 wherein thedigital baseband processor is configured to receive the indication ofreceived signal strength and to generate the first gain setting signalbased on the indication of received signal strength.
 6. The system ofclaim 5, wherein the digital baseband processor is further configured tocount a number of ADC outputs within a number of ADC outputs outside arange of values during a sliding time window; and wherein the secondgain setting signal is determined based on the number of ADC outputsoutside the range of values.
 7. The system of claim 2, wherein the mixerincludes: a plurality of mixers, each mixer configured to downconvert areceived signal about a different frequency, with an output of theplurality of mixers selectable using a band select signal.
 8. The systemof claim 7 wherein the second stage comprises a plurality of amplifierchains, with an amplifier chain of the plurality of amplifier chains foreach mixer, and wherein the second gain setting signal comprises aseparate gain setting signal for each amplification chain.
 9. A methodfor adjusting a gain imparted to a signal received at a receiver, themethod comprising: initializing a first gain setting to a maximum andinitializing a second gain signal to a maximum; amplifying a receivedsignal based on the first gain setting and the second gain setting;adjusting the first gain setting based on an indication of receivedsignal power; and adjusting the second gain setting based on valuesprovided by digitizing the amplified received signal.
 10. The method ofclaim 9 further comprising adjusting the second gain setting on a perfrequency subband basis.
 11. The method of claim 8, wherein adjustingthe second gain setting includes: comparing values provided bydigitizing the amplified received signal to a range of values; andincrementing or decrementing the second gain setting based on theresults of the comparison.
 12. A method for performing automatic gaincontrol during a signal processing of OFDM symbols in a received signalat an RF receiver, the OFDM symbols arriving at the RF receiver indifferent frequency subbands according to a pattern indicated by atime-frequency code (TFC) for frequency subband hopping, the RF receiverhaving a first gain, a second gain, and a third gain operating in serieson the OFDM symbols, the third gain including parallel gains, a numberof the parallel gains being equal to a number of the different frequencysubbands, each parallel gain corresponding to one of the frequencysubbands, each parallel gain having a gain corresponding to a gainindex, the method comprising: initializing the first gain, the secondgain, and the parallel gains of the third gain to a highest value foreach; amplifying the OFDM symbols in all of the frequency subbands bythe first gain and the second gain, amplifying the OFDM symbols in eachof the frequency subbands by the corresponding parallel gain; measuringan indication of a strength of the received signal to obtain a receivedsignal strength indication; obtaining a first-second gain combinationfrom a look-up table corresponding to the received signal strengthindication; adjusting the first gain and the second gain by valuescorresponding to the first-second gain combination; digitizing a resultof the third gain to obtain digitized outputs; counting a first numberof the digitized outputs having absolute values falling within a firstrange and a second number of digitized outputs having absolute valuesfalling above the first range; changing the gain index if the firstnumber and the second number exceed or fall below predetermined limits;and adjusting the third gain by adjusting each of the component gains tovalues corresponding to the changed gain index.
 13. The method of claim12, wherein each parallel gain of the third gain includes a plurality ofcomponent gains; wherein the component gains each have a plurality ofdiscrete gain values; and wherein unique combinations of the componentgain values is associated with the gain index.
 14. The method of claim12, wherein the different frequency subbands consist of a first subband,a second subband, and a third subband.
 15. The method of claim 12,wherein the OFDM symbols arriving in packets, each packet including aplurality of the OFDM symbols, each packet having a long preamble or ashort preamble, the short preamble including fewer OFDM symbols than thelong preamble, and a first arriving packet having the long preamble,wherein the first gain and the second gain are adjusted after each ofthe OFDM symbols occurring only during the long preamble, wherein thethird gain is adjusted during the long preamble after the adjusting ofthe first gain and the second gain, wherein the third gain is adjustedduring the short preamble.
 16. The method of claim 12, wherein the thirdgain is adjusted to maintain substantially 5% or less of the digitizedoutputs clipped to maximum absolute value.
 17. The method of claim 12,wherein the obtaining of a first-second gain combination from a look-uptable includes: obtaining an RF signal power by subtracting the firstgain and the second gain from a power of the received signal; and usingthe RF signal power to obtain the first-second gain combination from thelook-up table.
 18. An automatic gain control (AGC) method forfrequency-hopped OFDM ultra-wide band communication, OFDM symbolshopping over a plurality of frequency subbands, the method comprising:performing a coarse gain control during a first AGC stage; and refiningresults of the coarse gain control during a second AGC stage; whereinthe first AGC stage determines a common low noise amplifier gain and acommon mixer gain for all subbands within the plurality of subbands,wherein the first AGC stage updates the common low noise amplifier gainand the common mixer gain using received signal strength indicationmeasurements during the second AGC stage, wherein the first AGC stageaverages the common low noise amplifier gain and the common mixer gainover a first sliding window over one OFDM symbol, wherein the second AGCstage determines a programmable gain amplifier gain for each subbandwithin the plurality of subbands, wherein the second AGC stagedetermines the programmable gain amplifier gain by using a two-binhistogram measurement of output samples of an analog to digitalconverter for each subband within the plurality of subbands, wherein thesecond AGC stage averages the histogram measurement over a secondsliding window over one OFDM symbol, wherein the second AGC stagedetermines the programmable gain amplifier gain such that substantially5% of the output samples are clipped to maximal absolute value, whereinthe OFDM symbols are received in groups forming a packet, the packethaving a preamble including a plurality of OFDM symbols, wherein thefirst AGC stage and the second AGC stage are performed during receivingthe preamble, wherein the first AGC stage and the second AGC stage areperformed when the preamble is long, the long preamble including 24 OFDMsymbols, wherein only the second AGC stage is performed when thepreamble is short, the short preamble including 12 OFDM symbols, whereinthe first AGC stage and the second AGC stage are performed depending ona frequency band hopping pattern of the OFDM symbols being received,wherein the OFDM symbols are received over more than one antenna, andwherein the first AGC stage and the second AGC stage are performedseparately for the OFDM symbols received over each antenna.